Carotid or vertebrobasilar stenosis restricts distal blood flow, which decreases blood supply to the parts of the brain subserved by these vessels, and increases the risk of ischemic stroke. Surgical intervention with carotid endarterectomy or endovascular angioplasty/stenting is generally pursued if the diameter of the lumen of the internal carotid artery (ICA) is reduced more than 70%, which is typically documented by noninvasive imaging. Collateral circulation increases in the brain as a normal physiologic mechanism to by-pass and compensate for the blockage in the main artery. In some cases, this increased collateral flow can supply enough oxygenated blood to maintain adequate cerebral perfusion for supporting brain function in symptom free patients. The importance of adequate hemodynamic compensation via collateral circulation has been shown in patients with cerebral arterial stenosis.
Selectively labeled blood in individual arteries can be useful for a wide range of diagnostic purposes, including monitoring disease progression in the patient with collateral circulation. Digital subtraction angiography (DSA) is considered the gold standard for assessment of collateral circulation, and is the most widely used technique. However, the procedure is invasive and requires the use of ionizing radiation, as well as the injection of iodinated contrast media. Other non-invasive methods for direct assessment of collateral circulation are MRA and transcranial doppler ultrasound.
Focal arterial stenosis can be clinically evaluated using a variety of imaging methods, including duplex ultrasound, computed tomography angiogram (CTA), and magnetic resonance angiography (MRA). Although invasive CT-based methods have been used for qualitative assessment of vascular territory perfusion, quantitative mapping of blood flow from individual source arteries is still not practical in the clinical setting. Vascular territory mapping using arterial spin labeling (ASL) has been proposed, but currently typically requires complicated planning prior to scanning and extensive post-processing, which hinders the practical clinical use of these methods. Pseudo-continuous ASL (PCASL) tagging can be used for vessel-encoded ASL (VE-ASL) utilizing gradients applied during the tagging period to spatially encode multiple feeding arteries. See, e.g., Wong, M R M, 58: 1086-1091, 2007. A random encoding strategy was introduced for the detection of arteries without a priori knowledge of vessel locations. See, Wong & Guo, 19th ISMRM: 294, 2011. However, PCASL-based VE-ASL methods often require a long scan time and complicated clustering algorithms to classify multiple vascular territories. In addition, resolving mixed signals from multiple arteries can be problematic, particularly in clinical situations where a reasonable scan time and minimal manual intervention is desired for both acquisition and post-processing.